The following relates to the fabrication of composite structures using resin transfer molding (RTM) in areas such as high-tech composite structure fabrication.
Composite structures are well known for their physical properties of high strength and light weight materials. With these qualities, composite materials are gaining wide use in a variety of structural and non-structural applications. RTM is one method of fabricating composite structures.
Current RTM technology produces lightweight parts with excellent mechanical properties. As the use of composite parts become more common in aerospace and aviation, a need arises for the RTM process to achieve higher production rates.
Any high rate production method must maintain the desirable qualities of RTM composite parts while decreasing the cycle time or “takt time” of the forming process. The resin injector system that includes resin filling, cleaning, and processing is one area to examine for improvements.
RTM Process Description
The Resin Transfer Molding (RTM) process is advantageous because it can consistently produce composite parts with high strength, complex geometries, tight dimensional tolerances, and part quality typically required of aerospace applications. In the RTM fabrication process, a structure made up of reinforcing material, known as a preform, is placed in a closed matched mold which is then tightly sealed. A high vacuum is typically applied to the mold before and during injection to improve part quality and resin flow throughout the mold. Resin is then injected into the mold under elevated pressure and temperature to impregnate the preform. The impregnated preform structure is then cured to produce the final molded product.
In RTM and most other composite manufacturing processes, the final part geometry and mechanical strength properties are determined simultaneously. Composite structure applications that require high strength and tight geometric tolerances must have a fabrication process that controls several critical parameters including preform creation, injection, and curing.
One element of the RTM process is the resin injection system that is required to inject thermosetting resin at an elevated temperature and pressure into the mold. To achieve this the injection system resin container must be capable of both displacing resin and sealing resin from leakage at typical process temperatures of 250° F. (˜120° C.), injection pressures of 250 psi (˜1.7 MPa), and vacuum greater than 1 torr (˜100 Pa).
Positive Displacement Injectors
One of the most common methods of injecting resins for RTM fabrication is a positive displacement (PD) injection system. A PD injection system comprises a specialized cylinder into which resin is loaded. Located at the bottom of the cylinder is a movable piston. The piston is connected to an actuator which translates the piston up the cylinder displacing resin out and into the mold. The cylinder and piston assembly of a positive displacement injector is highly engineered to displace resin at elevated temperatures while under the high vacuums or high pressures required in the RTM process.
PD injection systems also allow for resin degassing processes to be conducted after the resin is loaded into the cylinder by sealing the cylinder and applying a vacuum. Positive displacement injectors provide precise control of resin pressure, flow rate, and temperature control critical to the RTM process. PD injections systems require the manual loading, degassing, and cleaning of resin which requires additional process time. While this is acceptable for low rate RTM production, typically 2-3 parts per day, the required injection processing time, manual operations, and operator exposure to resin and solvents may be unacceptable for higher production rates.
Pressure Pots
Another method of injecting resin for RTM molding involves the use of a pressure pot injection system. A pressure pot consists of a chamber which is filled with resin and tightly sealed. To inject resin, the chamber is pressurized with a compressed gas. This pressure forces resin into the inlet of a tube submerged in the resin and which exits at the mold. Pressure pots are capable of heating resin as well as sealing against vacuum for resin degassing operations.
Pressure pot injection system provide the pressure monitoring and control similar to PD injectors while typically being more compact and simple to operate. Pressure pots are typically disadvantaged when used to achieve the high strength and quality requirements in aerospace applications of RTM. This is due to a lack of accurate resin flow control critical to the RTM process. Pressure pots may also pose a high safety risk due to bursting of the chamber under the high (typically >250 psi (˜1.7 Mpa)) injection pressure required. Pressure pots like PD injectors require the manual loading, unloading, degassing, and cleaning of resins which requires additional process time. This requires additional injection processing time, manual operations, and operator exposure to resin and solvents that do not support higher production rates.
Pail Unloading Injectors
A third method of RTM injection may be performed with pail unloaders. Pail unloader injection systems are unique in that they are able to draw resin from the resin shipping container and inject directly into the RTM mold. A pail unloader accomplishes this by driving a heated punch into the resin shipping container. The punch heats the resin and displaces it into a geared pump. The pump then controls resin flow and pressure as it pumps resin to the mold.
Pail unloaders allow multiple injections to be performed from the single loading of a typical 5 gallon resin shipping container. The cleaning cycle is longer and more complex than PD or pressure pot injectors; however the cleaning is only performed once per pail loading, which aids in reducing the overall injection processing time.
Note that pail unloaders typically include a geared pump design that is particularly prone to large resin pressure and flow pulsations that are unacceptable in high performance RTM processing. Frequent cleaning is required to prevent the thermosetting resins from curing and to remove buildup inside the internal pump passageways. Such cleaning requires the use of large volumes of solvents for flushing the pump to clear resin from within the pump. The manual operations required, injection process fluctuations, and operator exposure to resin and solvents is unacceptable for higher production rates.
Various techniques may be used for RTM cleaning.
Pressure Pot Cleaning
One method of pressure pot cleaning is to scrape the remaining resin out of the pressure pot, then use solvent to clean the pot. A second method is to place a disposable liner in the pressure pot, then dispose of the liner after using the pressure pot. A third method is to place a resin container within the pressure pot. With this third method, the separate resin container is either cleaned or disposed.
Cleaning Positive Displacement Injectors
In positive displacement injectors, the injection cylinder and piston must be cleaned before another injection cycle can be conducted. The cylinder and piston are usually cleaned with solvent. The injector end cap is removed and the actuation rod pushes the piston all the way out of the cylinder. This action pushes the remaining resin out of the cylinder. The piston is removed from the actuation rod and the rod is retracted. The cylinder is wiped out with solvent and the piston is cleaned with solvent. Next, the actuation rod is extended so the piston can be attached. After reattaching the piston, it is drawn back into the cylinder. This method exposes workers to resin and solvent.
Cleaning Pail Unloaders
The heating punch is inserted into the top of the resin container to warm and pump the resin during injection. For cleaning, the heating punch is extracted from the resin container and placed in a similar container filled with solvent. The resin pump is activated. The pump draws solvent into the pump and circulates solvent through the pump and the resin injection lines. Solvent re-circulates for a period of time sufficient to clear the pump and lines of resin. The heating punch and wiper seal must also be cleaned with solvent. This method requires a significant amount of solvent.
The following comprise some limitations of current RTM methods.
Complex Time Consuming Operations
Current methods include complex operations for loading and unloading resin containers. Each individual process adds to the turn-around time or takt time of the injection process.
Time Consuming Cleaning Cycles
Frequent solvent-based cleanings are required to prevent buildup in pumps and containers.
Lack of Resin Degassing
Resin degassing capabilities are not available with some current methods.
Safety Problems—Exposing Workers to Resin and Solvent
Current methods expose workers to resin when handling resin containers, inserting tubing into resin containers and when cleaning containers. Cleaning procedures require the workers to use and dispose of solvents, which can be hazardous substances.
Process Variability
The heating and pumping systems employed in current methods cause variations in resin outlet pressure, flow rate, and resin temperature. These issues can affect the quality of parts produced.